Why REM sleep? Clues beyond the laboratory in a more challenging world

► REM has much in common with wakefulness and feeding, by actively preparing for wakefulness, rather than act as a recovery mode. ► Animal and human laboratory studies of REM are 'protective', seldom 'ecological', and can mask REM function in the 'wild' state. ► REM's atonia seems critical to REM, equating with waking locomotion, with both potentially interchangeable, to enhance brain plasticity. ► New findings outside sleep research, provide greater perspectives on REM.